Production of aluminum zinc magnesium alloy articles

ABSTRACT

IN PROCEDURE FOR PRODUCING A SOLUTION HEAT TREATED ALUMINUM-ZINC-MAGNESIUM ALLOY ARTICLE WHICH IS SUBJECTED TO SUBSTANTIAL COLD WORKING AFTER SOLUTION HEAT TREATMENT, BUT IS NEVERTHELESS CHARACTERIZED BY SUPERIOR RESISTANCE TO STRESS CORROSION, THE SUCCESSIVE STEPS OF HEATING THE ARTICLE TO A SOLUTION-HEAT-TREATING TEMPERATURE; COOLING THE ARTICLE, AT A RATE MAINTAINED BETWEEN 0.1*C./SECOND AND 20*C./SECOND AT LEAST DURING COOLING OF THE ARTICLE FROM 350*C. TO 250*C.; AND SUBJECTING THE ARTICLE TO MORE THAN 5% COLD WORKING. PREFERABLY, THE ALLOY CONTAINS 0.20-0.7% MANGANESE AND 0.01-0.25% ZIRCONIUM. A SPECCIFIC PREFERRED ALLOY COMPOSITION CONSISTS ESSENTIALLY OF 3.8-4.6% ZINC; 1.0-2.0% MAGNESIUM; 0.20-0.7% MANGANESE; 0.10-0.25% ZIRCONIUM; NOT MORE THAN 0.4% IRON; NOT MORE THAN 0.25% SILICON; OTHER ELEMENTS, NOT MORE THAN 0.03% EACH AND NOT MORE THAN 0.10% TOTAL; BALANCE ALUMINUM.

United States Patent 3,580,747 PRODUCTION OF ALUMINUM ZINC MAGNESIUM ALLOY ARTICLES Fred Howitt, Collins Bay, Ontario, and Ian Taylor, Kingston, Ontario, Canada, assiguors to Aluminium Laboratories Limited, Montreal, Quebec, Canada No Drawing. Filed Nov. 17, 1967, Ser. No. 683,782

Int. Cl. C22f N04 US. Cl. 14812.7 13 Claims ABSTRACT OF THE DISCLOSURE In procedure for producing a solution heat treated aluminum-zinc-magnesium alloy article which is subjected to substantial cold working after solution heat treatment, but is nevertheless characterized by superior resistance to stress corrosion, the successive steps of heating the article to a solution-heat-treating temperature; cooling the article, at a rate maintained between 0.1 C./ second and 20 C./second at least during cooling of the article from 350 C. to 250 C.; and subjecting the article to more than 5% cold working. Preferably, the alloy contains 0.20-0.7% manganese and 0.10-0.25% zirconium. A specific preferred alloy composition consists essentially of 3.8-4.6% zinc; 1.0-2.0% magnesium; 0.20-0.7% manganese; 0.10-0.25% zirconium; not more than 0.4% iron; not more than 0.25% silicon; other elements, not more than 0.03% each and not more than 0.10% total; balance aluminum.

BACKGROUND OF THE INVENTION This invention relates to the production of aluminumzinc-magnesium alloy articles which are solution heat treated and which may be subjected to substantial cold strain, as by cold working, after the solution heat treatment. In specific aspects, the invention is directed to procedures for producing aluminum-zinc-magnesium alloy articles which are subjected to substantial cold working following solution heat treatment, and to alloy compositions having particular utility in such procedures, these procedures and compositions affording in the produced articles superior resistance to stress corrosion.

Aluminum-zinc-magnesium alloys as herein contemplated are aluminum-based alloys wherein the principal alloying elements are zinc and magnesium (the major hardening constituent being a compound of magnesium and zinc), and which have a copper content of not more than a minor fraction of 1%. Various alloys of this type are known and have wide utility, for example in sheet, plate, extrusion and other wrought form. The aluminumzinc-magnesium alloys are heat treatable; especially as thus treated, such alloys are characterized by advantageously high mechanical strength.

Accordingly, workpieces of these alloys are commonly subjected to a so-called solution heat treatment, as well as to subsequent natural and/ or artificial age hardening steps for which the heat treatment is a necessary prerequisite, in order to improve the alloy strength and hardness. Conventional solution heat treatments include the successive steps of heating the workpiece to bring it to a temperature high enough to produce complete solid solution of the alloy constituents but too low to produce incipient fusion, soaking the workpiece by maintaining it at such temperature for a predetermined finite time to effect the desired complete solution of constituents, and quenching the workpiece at a relatively rapid rate in a suitable cooling medium, e.g. water.

In many instances of use of aluminum-zinc-magnesium alloys, it would be desirable to subject the alloy work- 3,580,747 Patented May 25, 1971 piece to substantial cold Working after solution heat treatment; and indeed, it is commonly necessary to apply some degree of cold deformation to solution heat treated articles, as for rectification of shape or to increase tensile strength, or by operations such as cold bending, shearing, drawing or forming, all of which result in residual cold strain. However, cold strain applied after conventional solution heat treatment (including a rapid quench) to a workpiece fabricated of an aluminum-zinc-magnesium alloy composition as heretofore known tends to render the workpiece susceptible to stress corrosion cracking, a condition which occurs when the alloy article is subjected to stress during use and exposed to a corrosive environment, the cracks developing at the localities of strain in the article. For this reason, in prior practice, application of substantial cold Working to a solution heat treated aluminum-zinc-magnesium alloy article has been avoided.

To some extent, aluminum-zinc-magnesium alloys which have been solution heat treated are susceptible to stress corrosion even if not subsequently cold worked, and various procedures and alloy compositions have been proposed for the purpose of minimizing this susceptibility. For example, the addition to the alloy of minor amounts of metals including chromium, and subjection of the solution-heat-treated workpiece to a special two-step artificial age-hardening operation, have been suggested in U-S. Pat. No. 3,171,760; but these expedients are not directed to improvement of stress corrosion resistance in workpieces which undergo substantial cold working after solution heat treatment.

SUMMARY OF THE INVENTION An object of the invention is to provide superior resistance to stress corrosion in wrought aluminum-zincmagnesium alloy articles which are subjected successively to solution heat treatment and substantial cold Workmg.

To this and other ends, the invention in one aspect broadly contemplates procedure for producing such articles, comprising in succession the steps of heating an aluminum-zinc-magnesium alloy workpiece to a temperature sufficiently high to elfect substantial solution of alloy constituents therein, cooling the workpiece from the last-mentioned temperature at a rate of between about 0.1 C. and about 20 C. per second at least during the cooling of the workpiece from about 350 C. to about 250 C., and subjecting the cooled workpiece to cold working of more than 5% To illustrate the meaning of percentage values of cold working as herein given, reference may be made (by way of example) to the cold rolling of aluminum alloy sheet. In such case, percent cold working means percent cold reduction of the sheet. It will be understood, however, that the invention is not limited either to the treatment of sheet or to cold working by rolling, but embraces treatment of other articles and cold working operations other than rolling, e.g. stretching, bending and twisting.

It is found that by this procedure greatly superior resistance to stress corrosion cracking is achieved in the produced article, notwithstanding subjection of the article to substantial cold working after solution heat treatment. In particular, the present procedure affords production of articles subjected to very substantial cold working-10% or above-after solution heat treatment and nevertheless characterized by very advantageously low stress corrosion susceptibility. As presently believed, the observed improvement in stress corrosion resistance is attributable particularly to the step of cooling from solution-heat-treating temperature at the indicated low rate over the range of temperatures between 350 and 250 C.

For attainment of optimum stress corrosion resistance, it is preferred that the cooling rate (at least between 350 C. and 250 C.) be not more than about 12 C. per second, and indeed the cooling step may very advantageously be performed by exposing the heated workpiece to still air, which effects cooling for most common thicknesses of sheet at a rate of between about 1 and about 6 C. per second.

Further in accordance with the invention, it is preferred that the alloy used contain about 0.20 to about 0.7% manganese, and about 0.10 to about 0.25% zirconium, inclusion of these two metals in the stated ranges being found to afford special improvement in stress corrosion resistance of the ultimately produced article. When alloys having this preferred composition are employed, fully satisfactory stress corrosion resistance is obtained with cooling rates (i.e. between 350 and 250 C.) as high as and indeed even higher than the upper limit of 20 C. per second, whereas alloys lacking the combination of manganese and zirconium inclusions exhibit greatest resistance to stress corrosion when cooled (over the 350- 250 C. range) at rates provided by exposure to still air.

In addition, it is preferred that the alloy used contain not more than 0.05% chromium. Presence of chromium in excess of this amount appears to promote exfoliation of the produced article and also increases the sensitivity of mechanical properties to cooling rate.

In a further aspect, the invention contemplates the provision of an aluminum-zinc-magnesium alloy especially suitable for use in the foregoing procedure and consisting essentially of about 3.8 to 4.6% zinc; about 1.0 to 3.0% magnesium; about 0.20 to 0.7% manganese; about 0.10 to 0.25% zirconium; not more than about 0.40% iron; not more than about 0.25% silicon; other elements, each not more than about 0.03%, total not more than 0.10%; balance aluminum. Articles fabricated of this alloy are found to exhibit a markedly superior resistance to stress corrosion when treated by the procedure described above.

Stated more generally, it is found that solution heat treated articles fabricated of the above-described alloy may be subjected to cold strain without developing stress the range of temperatures between 350 C. and 250 C.)

be bet-ween about 0.1 C. per second and about 20 C. per second, but such workpieces exhibit satisfactory resistance to stress corrosion (i.e. notwithstanding subjection to cold strain after solution heat treatment) even when quenched from solution heat treatment rates up to about 80 C. per second through the stated range of temperatures; and indeed in many cases still higher cooling rates may be employed in treatment of this alloy without producing deleterious stress corrosion susceptibility upon subsequent cold strain.

Further features and advantages of the invention will be apparent from the detailed description hereinbelow set forth.

DETAILED DESCRIPTION The method of the invention may be described as embodied in procedures for treating workpieces constituted of aluminum-zinc-magnesium alloys having the following composition: not more than about 0.35% silicon, not more than about 0.40% iron, not more than about 0.20% copper, not more than about 0.9% manganese, about 0.5 to about 3.0% magnesium, not more than about 0.25% chromium, about 3.6 to about 5.0% zinc, not more than about 0.15% titanium, not more than about 0.30% zircom'um, balance aluminum, with impurities present in amounts of not more than about 0.05 each and 0.15% total; An example of a known alloy of the foregoing type is the alloy designated by Aluminum Association No. X7004 (also sometimes commercially known as 748 alloy), which has the following composition: Si, not more than 0.25 Fe, not more than 0.40% Cu, not more than 0.20%; Mn, 0.40-0.9%; Mg, 1.02.0%; Cr, not more 4 than 0.25%; Zn, 1.0-4.6%; Ti, not more than 0.15%; impurities, not more than 0.05 each, not more than 0.15 total; balance aluminum.

In accordance with the procedure of the present invention, a workpiece fabricated of an alloy having the general composition set forth above is subjected to successive steps of solution heat treatment, cooling and cold working. By way of example, the workpiece may be a strip of alloy sheet, although it will be understood that the invention broadly embraces treatment of other types of wrought articles as well. The solution heat treatment step is performed by heating the workpiece sufliciently to effect at least substantial solution of the alloy constituents therein, as will be understood by those skilled in the art. Specifically, in this heating step the workpiece is brought to a maximum temperature which is in a range between about 350 C. and about 550 C. Thus, the workpiece may be placed in or advanced through a suitable furnace for heating to effect such solution of constituents, being brought to a temperature of e.g. 465 C. Alternatively, the heating step may be performed incident to other operations on the workpiece, such as extrusion or hot rolling, i.e. the heating of the workpiece for these operations may serve to elfect solution of alloy constituents and thus may constitute the heating step of the present procedure, with out resort to a separate and special heat treatment of the workpiece. 1

Further in accordance with the invention, the heated workpiece is cooled from the solution-heat-treating step at least through the range of temperatures from about 350 C. to about 250 C; at a rate of between about 0.1 C. and about 20 C. per second. Preferably, the cooling rate is at least about 05 C. per second; at lower rates some impairment of mechanical properties in the treated article is observed, but the tensile strength developed with cooling rates as low as 0.1 C. per second is still acceptable. This cooling may be carried out in any convenient manner, as with water sprays or jets of air or other gas. In many instances it is preferable that the cooling rate (at least through the range of temperatures above specified) be not more than about 12 C. per second, and indeed it is convenient and advantageous to effect the cooling of the workpiece by exposing it to still air. The cooling rate of alloy articles of various thicknesses in still air (over the range 350-250 C.) as compared with the cooling rates produced by quenching in C. water and in 18 C. water, are given in the following table:

Cooling rate at various media C./sec.)

Still further in accordance with the present method, after the workpiece has been cooled it is cold worked to any desired extent in excess of 5%. For example, in the production of alloy sheet, the workpiece may be rolled to an intermediate gauge before solution heat treatment, and after cooling following the solution heat treatment, it may be further reduced in thickness in an amount of above 5%. It will be understood, however, that the invention also embraces application of cold strain by working operations other than rolling.

By the performance of the above-described succession of steps, there is produced a workpiece which is high y resistant to stress corrosion despite the substantial amount of cold working to which it is subjected after solution heat treatment. That is to say, the resistance of such a workpiece to stress corrosion is far greater than that of a workpiece similarly solution-heat-treated and subsequently coldworked butcooled after solution heat treatment by conventional quenching in water at a rate of about 2,000 C. per second; this improvement is realized for all degrees of cold working within the stated range, although in ordinary cold working operations, the practicable limit for cold reduction is usually about 80%. Thus in one specific aspect, the invention particularly contemplates procedures wherein the article under treatment is subjected to cold working of at least about after solution heat treatment (e.g. in the case of cold rolling of sheet, cold reduction of at least about 10% In particular, the increased resistance to stress corrosion afforded by the present method enables use of the product in many applications for which articles produced by conventional procedures, with conventional quenching after solution heat treatment, are not satisfactory owing to rapid stress corrosion failure. As already stated, the advantages achieved in this regard are believed attributable to the very low quenching rate used, in the combination with solution heat treatment. Stated generally, the stress corrosion resistance of articles produced by the present method decreases as the cooling rate after solution heat treatment is increased above the upperlimit of 20 C. per second, although alloys having the preferred composition of the invention (described below) are less sensitive to variations in quenching rate above this limit than are other aluminum-zinc magnesium alloys.

Notwithstanding the very low quench rate used in the present method, the solution heat treatment is fully effective to develop desired mechanical properties in the treated workpiece; i.e. no significant differences are found between the tensile and bend properties attained in a workpiece cooled at the low rate of this invention, and those developed in workpieces which are cooled after solution heat treatment by a conventional fast water quench.

In the present method, the workpiece may be naturally aged (i.e. stored at room temperature, without application of heat) for any desired period before and/or after the cold working step. No artificial aging is necessary prior to cold working in order to develop desired stress corrosion resistance, and it is convenient to perform the cold working operation without intervening artificial aging after the solution heat treatment. Nevertheless, the workpiece may if desired be artificially aged (e.g. in conventional manner, by heating to and holding at a suitable elevated temperature as will be apparent to those skilled in the art) before and/or after the cold working step.

While the foregoing method is applicable generally to alloys of the aluminum-zinc-magnesium type as exemplified by X7004 alloy mentioned above, it is especially preferred that the alloy employed consist essentially of about 3.8 to 4.6% zinc, about 1.0 to 2.0% magnesium, about 0.20 to 0.7% manganese, about 0.10 to 0.25% zirconium, not more than about 0.40% iron, not more than about 0.25% silicon, other elements, each not more than about 0.03%, total not more than 0.10%, balance aluminum. The alloy having this composition, which constitutes a further and particular feature of the invention, exhibits especially high resistance to stress corrosion (when solution heat treated, cooled and cold worked in accordance with the method of the invention) and has a correspondingly reduced sensitivity to quenching rate, i.e. so that very superior stress corrosion resistance is achieved even when the alloy is cooled after solution heat treatment at a rate as high as 20 C. per second, and indeed at rates as high as 80 C. per second or even higher. This result is believed attributable to the inclusion of manganese and zirconium in the preferred composition. Absence of chromium from the composition (except as an impurity, within the limit of 0.03% for any such impurity) is also advantageous since the presence of greater amounts of chromium (e.g. above 0.05%) tends to promote exfoliation and consequent failure of the alloy article under corrosion-producing conditions while at the same time increasing the sensitivity of the mechanical properties of the alloy to cooling rate after solution heat treatment.

Stated more generally, it is found that workpieces fabricated of an alloy having the composition just set forth may be subjected to cold strain (e.g. by cold working) after solution heat treatment without developing significant stress corrosion susceptibility when cooled from solution heat treatment (at least through the range of temperatures from 350 C. to 250 C.) at rates as high as C. per second or even higher. That is to say, the alloy composition of the invention provides increased stress corrosion resistance in solution heat treated and subsequently cold strained articles, and this improvement is enhanced by cooling through the 350- 250 C. range at a rate of not more than about 80 C. per second and preferably not more than about 20 C. per second.

Presently preferred ranges and nominal values of proportions of elements in the alloy composition of the invention are set forth in the following table:

Range or Maximum Nominal (percent) (percent) By way of further and more specific illustration of the invention, reference may be had to the following specific examples:

EXAMPLE I A plurality of aluminum-zinc-magnesium alloy sheets of various gauges were produced by rolling an alloy having the following approximate composition: 4.4% zinc, 1.8% magnesium, 0.71% manganese, 0.26% iron, 0.15% silicon, 0.03% copper, 0.018% titanium, balance aluminum. These sheets were solution heat treated by soaking for one hour at 465 C. One group of the sheets was cooled after solution heat treatment by quenching in cold (10 C.) water, and the remaining group of sheets was cooled from solution heat treating temperature by exposure to still air. The approximate cooling rates for the various sheet thicknesses in the two cooling media used, over the range of temperatures from 450 C. down to 250 C., were as follows:

Thickness Air cooled Water quenched Cooling from solution heat treatment: cold Water quench,

air cooling;

Duration of natural aging before cold reduction: 1, 35,

365 days;

Extent of cold reduction: 0, 20, 60%;

Duration of natural aging after cold reduction: 1, 35,

days.

At the conclusion of the indicated natural aging periods, five strips each approximately 0.75 inch wide and 6 inches long were cut from each sample (with the long dimension of each strip transverse to the direction of rolling),

and each strip was bent to an are by compressing the ends between grooves in alinen-filled Bakelite strip, the degree of bend being such that a small amount of plastic deformation occurred in the outer fibers of the side of the strip in tension. Each bent strip was exposed to an aqueous solution of 1 normal NaCl+0.2 normal H the solution was monitored daily to maintain the available oxygen at a constant level. Specifically, each strip was alternately exposed to the solution for 16 hours and air dried for 8 hours (on working days but not on weekends), and stress corrosion failure of the strips was determined by daily visual inspection. Strips which survived 100 days of such exposure without stress corrosion failure were considered insensitive to stress corrosion and were removed from test. Other corrosion effects such as pitting and exfoliation become noticeable at this time.

The foregoing test for stress corrosion, commonly referred to as a bent strip alternate immersion test, essentially follows the procedure described in the A.S.T.M. Symposium on Stress Corrosion Testing, No. 64, published in 1944..

Comparative results of the stress corrosion test for air cooled and water quenched samples reduced 20% and 60% by cold rolling after solution heat treatment are given in the following table. In this table, the stress corrosion resistance of each set of five strips is represented by the lifetime under test (i.e. duration of exposure, in days, before stress corrosion failure) of the first strip of the set to fail (columns headed A) and the cumulative lifetime under test of the five strips (columns headed B). Aging times given refer to time delays between quenching and cold rolling. NF means that no failure occurred in 100 days of exposure. Values of strip lifetime marked with an asterisk refer to failures resulting from exfoliation.

aged after solution heat treatment but,

Ultimate tensile 0.2% yield strength strength Percent ,000 ,000 elongation Alloy temper p.s.1.) p.s.i.) (2) T3 reduction) 56 44 13 10 T3 (60% reduction) 71 66 a T4 (longitudinal properties 55 32 20 T4 (transverse properties). 55 32 19 T6 (longitudinal properties). 59 51 12 T6 (transverse pr0perties) 58 50 12 EXAMPLE II To illustrate the effect of alloy composition and cooling rate on stress corrosion susceptibility of aluminumzinc-magnesium alloy sheet subjected to substantial cold working after solution heat treatment, sheets were prepared from three alloys having the following composition:

Percent Zn Mg. Mn Fe Si Zr A1 1.7 0.31 0.26 0.16 -Balance. 1.8 0.71 0.26 0.16 D0. 1.8 0.28 0.23 0.16 0.14 Do.

' each, 0.10% total.

Lifetime under test before stress-corrosion failure (days) Water-quenched samples, aged- Air-cooled samples, aged Cold 1 day Days 100 Days 1 Day 35 Days 100 Days Time delay between rollreduction ing and testing (days) (percent) A B A B A B A B A B A B 1 20 NF NF 1 117 3 15 NF NF NF NF NF NF 23 166 6 60 5 67 *58 403 NF NF NF NF 35 20 62 462 2 11 2 10 NF NF NF NF NF NF 6o 27 144 4 29 1 10 *5s 426 *79 479 NF NF 365 20 78 478 2 143 1 29 NF NF NF NF NF NF e 3 15 NF NF 80 4s *74 474 8 81 34 Mean time to iailure=26. 3 days From the foregoing table, it will be seen that the samples which were air cooled after solution heat treatment exhibited for greater resistance to stress corrosion failure than the samples that were quenched in water. The failures that did occur in the group of air-cooled samples resulted from exfoliation.

In the case of samples which were not subjected to cold reduction after solution heat treatment, neither aircooled nor water-quenched samples exhibited stress corrosion failure within the 100-day testing period.

There was little or no difference between the mechanical properties of the air cooled and water quenched samples. Measured values for these properties are given in the following table, wherein T3 refers to the temper of samples cold worked after solution heat treatment and natural aging, T4 designates the temper of samples naturally aged for 35 days but not subjected to cold reduction, and T6 designates the temper of samples artificially 0 No stress OOZ'HZiSlOII failures in ays Sheets of 0.064 inch gauge were produced by rolling direct cast ingots of each alloy. A plurality of sheets of each alloy were solution heat treated. Twosheets of each alloy were air cooled after solution heat treatment ata rate of about 1.9 C. per second, another two sheets of each alloy were quenched with water sprays providing a cooling rate of about 20 C. per second, and still another two sheets of each alloy were quenched in water at a rate of about 1,100 C. persecond, the cooling rates in each instance being given for the range of temperatures from 350 C. to 250 C. One sheet of each pair was naturally aged for 35 days and the other for 100 days; at. theend of the specified natural aging times, each sheet was reduced 50% by cold rolling to a final gauge of 0.032 inch. Five strips cut from each sheet were then subjected to the stress corrosion test described in Example I above. The results of this test are set forth in the following table, wherein the various letters and symbols used (AK-B,

NP, and asterisk) have the same significance as in the table summarizing stress corrosion test results in Example 1:

The foregoing results demonstrate that the samples of alloy 3 (representing the composition of the present invention) exhibit very markedly superior resistance to stress corrosion, even in the case of sheets cooled at relatively rapid rates after solution heat treatment.

It is to be understood that the invention is not limited to the specific procedures and embodiments hereinabove set forth but may be carried out in other ways without departure from its spirit.

We claim:

1. In procedure for treating a workpiece constituted of an aluminum-zinc-magnesium alloy, the steps of successively (a) heating the workpiece to a temperature sufficiently high to effect substantial solution of alloy constituents therein;

(b) cooling said workpiece from said temperature, said cooling being performed at a rate of between about 01 C. per second and about C. per second at least while said workpiece is cooled from about 350 C. to about 250 C.;

(c) subjecting the cooled workpiece to more htan 5% cold working after cooling said workpiece as aforesaid; and

(d) aging said workpiece for hardening the alloy after cooling said workpiece as aforesaid;

(e) while maintaining said workpiece continuously at a temperature substantially lower than solution heating temperature for said alloy after cooling said workpiece as aforesaid and throughout the cold working and aging steps.

2. Procedure according to claim 1, wherein said temperature to which said workpiece is heated is between about 350 C. and about 550 C.

3. Procedure according to claim 1, wherein said rate of cooling the workpiece is at least about 0.5 C. per second.

4. Procedure according to claim 1, wherein said rate of cooling the workpiece is not more than about 12 C. per second.

5. Procedure according to claim 1, wherein the step of cooling the workpiece is performed by exposing the workpiece to air.

6. Procedure according to claim 1, wherein said workpiece is cooled during the cooling step down to substantially room temperature and is thereafter maintained 10 substantially at room temperature until completion of the cold working step.

7. Procedure according to claim 1, wherein said alloy contains about 0.20 to about 0.7% manganese and about 0.10 to 0.25% zirconium.

8. Procedure according to claim 7, wherein said alloy contains not more than about 0.05% chromium.

9. Procedure according to claim 8, wherein said alloy consists essentially of 3.8 to 4.6% zinc; 1.0 to 2.0% magnesium; 0.20 to 0.7% manganese; 0.10 to 0.25% zirconium; not more than 0.4% iron; not more than 0.25% silicon; other elements not more than 0.03% each and not more than 0.10% total; balance aluminum.

10. Procedure according to claim 1, wherein said workpiece is a sheet of said alloy and wherein said step of subjecting the cooled workpiece to cold working comprises reducing the sheet in thickness by cold rolling.

11. Procedure according to claim 1, wherein said cooled workpiece is subjected to at least about 10% cold working.

12. In procedure for treating a workpiece constituted of an alloy consisting essentially of 3.8 to 4.6% zinc; 1.0 to 2.0% magnesium; 0.20 to 0.7% manganese; 0.10 to 0.25 zirconium; not more than 0.4% iron; not more than 0.25 silicon; other elements not more than 0.05

each and not more than 0.15% total; balance aluminum, the steps of successively (a) heating the workpiece for effecting substantial solution of alloy constituents therein, to a maximum temperature between about 350 C. and about 550 C.

(b) cooling said workpiece from said temperature, said cooling being performed at a rate of between about 01 C. per second and about 80 C. per second at least while said workpiece is cooled from about 350 C. to about 250 C.;

(c) subjecting the cooled workpiece to more than 5% cold working after cooling said workpiece as aforesaid; and

(d) aging said workpiece for hardening the alloy after cooling said workpiece as aforesaid;

-(e) while maintaining said workpiece continuously at a temperature substantially lower than solution heating temperature for said alloy after cooling said workpiece as aforesaid and throughout the cold working and aging steps.

13. Procedure according to claim 11, wherein said rate of cooling the workpiece is not more than about 20 C. per second.

References Cited UNITED STATES PATENTS 2,245,166 6/1941 Stroup 148-11.5X 3,287,185 11/1966 Vachet et al. 146X 3,304,209 2/1967 Anderson et al. 75146X 3,306,787 2/1967 Dies 14811.5X 3,322,533 5/1967 Martin 75l46 3,329,537 7/1967 Loach 148--11.5X 3,332,773 7/1967 Dudas et a1. 75146 3,379,580 4/1968 Zei'gler 148--11.5 3,392,062 7/1968 Altenpohl et al. 14811.5 3,418,177 12/1968 Pryor 14811.5 3,464,866 9/1969 Pryor 148-115 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner U.S. Cl. X.R. 148-159 mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,SBO JMT Dated May 25 1971 Invent Fred Howitt and Ian Tavlor It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, lines 5 and 6, "Aluminium Laboratories Limited" (the assignee of the patent) should read --Alcan Research and Development Limited-- Col. 7, line 60, after "exhibited; "for" should read .--far-- Col. 8, line 3, "0.3%" should read --0.03%-- Col. 9, line 45, "htan" should read --tha.n--

Signed and sealed this 8th day of August 1972.

(SEAL) Attest:

EDWARD M.FLETCHEIR,JTR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

